The Collaborative International Dictionary of English v.0.48:

Twelfth \Twelfth\ (tw[e^]lfth), a. [For twelft, OE. twelfte, AS.
   twelfta. See Twelve.]
   1. Next in order after the eleventh; coming after eleven
      others; -- the ordinal of twelve.
      [1913 Webster]

   2. Constituting, or being one of, twelve equal parts into
      which anything is divided.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twelfth \Twelfth\, n.
   1. The quotient of a unit divided by twelve; one of twelve
      equal parts of one whole.
      [1913 Webster]

   2. The next in order after the eleventh.
      [1913 Webster]

   3. (Mus.) An interval comprising an octave and a fifth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

twelfth
    adj 1: coming next after the eleventh and just before the
           thirteenth in position [syn: twelfth, 12th]
    n 1: position 12 in a countable series of things
    2: one part in twelve equal parts [syn: one-twelfth,
       twelfth, twelfth part, duodecimal]